Formation of holes is an essential part of the formation of printed circuit boards. Drilled holes typically are plated with an electrically conducting material. The holes then may or may not receive conductors, for example pins, attached to chips, chip holders, or other components to be attached to the circuit board.
The electrically conducting plating material is electrically connected to electrically conducting planes within the circuit board. Therefore, when the conductors of the chips, chip holders, or other components are attached to the plated holes, the holes provide electrical connections among the chips, chip holders, other components of the board, and/or the device that the circuit board is incorporated into or other components incorporated into the device. Accordingly, it is important that the holes be drilled with great accuracy and are uniform in their dimensions. Variations in the surface of the hole or location of the hole could result in poor quality plating, poor, missing, or misplaced electrical connections, and ill fitting components, among other things.
Poor hole quality may result from many causes. The causes of poor hole quality encountered in drilling holes in circuit boards include damage caused by drill bit breakage, damage caused by impact of material removed by the drill, in particular damage from copper chips removed from electrically conducting layers of the circuit board such debris can buckle and gouge the wall of the hole being drilled. Additionally, the compaction of debris in the drill flute and the drill itself have been known to cause heating of the wall and, thereby softening of the wall, also leading to poor hole quality.
Another component of the problems encountered in drill holes in printed circuit boards is drill "smear". Drill smear occurs when a layer of resin from the circuit board is redeposited on the hole wall. This smear can be difficult to remove, especially if excessive. Drill smear can lead to unreliable connections between the subsequently applied plating on the hole wall and the conductive planes in the printed circuit board. This heating and smearing are particularly acute in high aspect ratio holes (holes having a high length to diameter ratio) because the chips have to travel a long distance rubbing against the hole wall before they are finally evacuated.
The problem of drill breakage not only may cause damage to the circuit board, but is a significant problem in and of itself. Drill bit damage costs money to replace broken drill bits and damaged circuit boards and time in replacing the drill bit, among other problems.
Typical drill bits currently used in forming holes in printed circuit boards include two flutes.
Typical solutions presently employed to address problems encountered in drilling high aspect ratio holes include peck drilling. According to the peck drilling method, a hole is drilled step-wise by applying the drill to the circuit board and removing some material, thereby starting a hole. Then, the drill is retracted from the hole. Next, the drill reenters the hole and removes more material. The drill then retracts from the hole again. This process is repeated until a hole of the desired depth is formed.
Only engaging the circuit board material for a brief period of time helps to prevent a compaction in the flute of chips of the circuit board material removed by the drill. However, peck drilling requires a great deal of time since drilling each hole requires the drill and/or the circuit board to be moved a plurality of times to drill each hole. Peck drilling has also failed to produce holes of sufficient quality in some circuit board materials.
Accordingly, the present inventors recognized the desirability of developing a drill bit that could drill holes of a desired quality in any circuit board, regardless of the material used to form the circuit board, in a single pass.